sedimentary
Ironstone Concretion
Siderite or Limonite Concretion (FeCO3 or FeO(OH)·nH2O)
Hardness: 4-5.5 (Mohs scale), Color: Rusty brown, tan, or grey, Luster: Dull/Earthy, Crystal Structure: Microcrystalline, Cleavage: None, Specific Gravity: 3.0-3.9
- Hardness
- 4-5
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Physical properties
Hardness: 4-5.5 (Mohs scale), Color: Rusty brown, tan, or grey, Luster: Dull/Earthy, Crystal Structure: Microcrystalline, Cleavage: None, Specific Gravity: 3.0-3.9
Formation & geological history
Formed by the precipitation of mineral cement (iron oxides or carbonates) within the spaces between sediment grains, often around a nucleus like a leaf, shell, or fossil, during early diagenesis.
Uses & applications
Historically used as a low-grade iron ore; currently used primarily as aquarium decor, garden stones, or for geological collecting.
Geological facts
Concretions are often mistaken for meteorites or dinosaur eggs due to their unusual spherical or elliptical shapes. The famous 'Moqui Marbles' in Utah are a specific type of iron oxide concretion.
Field identification & locations
Identify by its rounded shape, heavy weight relative to size (due to iron content), and characteristic rusty weathered exterior. Commonly found in shale and sandstone beds.
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